Modifying the Substrate-Dependent Pd/Fe2O3 Catalyst-Support Synergism with ZnO Atomic Layer Deposition.

Low-loading Pd supported on Fe2O3 nanoparticles was synthesized. A common nanocatalyst system with previously reported synergistic enhancement of reactivity that is attributed to the electronic interactions between Pd and the Fe2O3 support. Fe2O3-selective precoalescence overcoating with ZnO atomic layer deposition (ALD), using Zn(CH2CH3)2 and H2O as precursors, dampens competitive hydrogenation reactivity at Fe2O3-based sites. The result is enhanced efficiency at the low-loading but high reactivity Pd sites. While this increases catalyst efficiency toward most aqueous redox reactions tested, it suppresses reactivity toward polyaromatic core substrates. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) show minimal electronic impacts for the ZnO overcoat on the Pd particles, implying a predominantly physical site blocking effect as the reason for the modified reactivity. This serves as a proof-of-concept of not only stabilizing supported nanocatalysts but also altering reactivity with ultrathin ALD overcoats. The results point to a facile ALD route for selective enhancement of reactivity for low-loading Pd-based supported nanocatalysts.

Ultrathin Atomic Layer Deposited Al2O3 Overcoat Stabilizes Al2O3-Pt/Ni-Foam Hydrogenation Catalysts.

Galvanic exchange seeds the growth of Pt nanostructures on the Ni foam monolith. Subsequent atomic layer deposition of ultrathin Al2O3 followed by annealing under air affords supported Pt catalysts with ultralow loading (0.020 ppm). In addition to the expected enhancement of the stability of the Pt particles on the surface, the ∼2 nm Al2O3 overcoat appears to also play a crucial role in the overall structural integrity of the NiOx nanoplates that grow on the Ni foam surface as a result of the preparative route. The resulting material is physically robust toward repeated handling and showcases retention of catalytic activity over 10 standard catalyst recycling trials, standing in marked contrast to the uncoated samples. Catalyst activity was tested via the hydrogenation of various functionalized styrenes at low temperatures and low hydrogen pressure in ethanol as a solvent, with a TOF as high as 9.5 × 106 h-1 for unfunctionalized styrene. Notably, the catalysts show excellent tolerance toward F, Cl, and Br substituents and no hydrogenation of the aromatic ring.

Molybdenum(III) Amidinate: Synthesis, Characterization, and Vapor Phase Growth of Mo-Based Materials.

The synthesis, characterization, and thermogravimetric analysis of tris(N,N'-di-isopropylacetamidinate)molybdenum(III), Mo(iPr-AMD)3, are reported. Mo(iPr-AMD)3 is a rare example of a homoleptic mononuclear complex of molybdenum(III) and fills a longstanding gap in the literature of transition metal(III) trisamidinate complexes. Thermogravimetric analysis (TGA) reveals excellent volatilization at elevated temperatures, pointing to potential applications as a vapor phase precursor for higher temperature atomic layer deposition (ALD), or chemical vapor deposition (CVD) growth of Mo-based materials. The measured TGA temperature window was 200-314 °C for samples in the 3-20 mg range. To validate the utility of Mo(iPr-AMD)3, we demonstrate aerosol-assisted CVD growth of MoO3 from benzonitrile solutions of Mo(iPr-AMD)3 at 500 °C using compressed air as the carrier gas. The resulting films are characterized by X-ray photoelectron spectroscopy, X-ray diffraction, and Raman spectroscopy. We further demonstrate the potential for ALD growth at 200 °C with a Mo(iPr-AMD)3/Ar purge/300 W O2 plasma/Ar purge sequence, yielding ultrathin films which retain a nitride/oxynitride component. Our results highlight the broad scope utility and potential of Mo(iPr-AMD)3 as a stable, high-temperature precursor for both CVD and ALD processes.

Release Rate Studies of 5-Aminosalacylic Acid Coated with Atomic Layer-Deposited Al2O3 and ZnO in an Acidic Environment.

5-Aminosalicylic acid (5-ASA) is a first-line defense drug used to treat mild cases of inflammatory bowel disease. When administered orally, the active pharmaceutical ingredient is released throughout the gastrointestinal tract relieving chronic inflammation. However, delayed and targeted released systems for 5-ASA to achieve optimal dose volumes in acidic environments remain a challenge. Here, we demonstrate the application of atomic layer deposition (ALD) as a technique to synthesize nanoscale coatings on 5-ASA to control its release in acidic media. ALD Al2O3 (38.0 nm) and ZnO (24.7 nm) films were deposited on 1 g batch powders of 5-ASA in a rotatory thermal ALD system. Fourier transform infrared spectroscopy, scanning electron microscopy, and scanning/transmission electron microscopy establish the interfacial chemistry and conformal nature of ALD coating over the 5-ASA particles. While Al2O3 forms a sharp interface with 5-ASA, ZnO appears to diffuse inside 5-ASA. The release of 5-ASA is studied in a pH 4 solution via UV-vis spectroscopy. Dynamic stirring, mimicking gut peristalsis, causes mechanical attrition of the Al2O3-coated particles, thereby releasing 5-ASA. However, under static conditions lasting 5000 s, the Al2O3-coated particles release only 17.5% 5-ASA compared to 100% release with the ZnO coating. Quartz crystal microbalance-based etch studies confirm the stability of Al2O3 in pH 4 media, where the ZnO films etch 41× faster than Al2O3. Such results are significant in achieving a nanoscale coating-based drug delivery system for 5-ASA with controlled release in acidic environments.

ALD based nanostructured zinc oxide coated antiviral silk fabric.

The COVID-19 pandemic has underscored the importance of research and development in maintaining public health. Facing unprecedented challenges, the scientific community developed antiviral drugs, virucides, and vaccines to combat the infection within the past two years. However, an ever-increasing list of highly infectious SARS-CoV-2 variants (gamma, delta, omicron, and now ba.2 stealth) has exacerbated the problem: again raising the issues of infection prevention strategies and the efficacy of personal protective equipment (PPE). Against this backdrop, we report an antimicrobial fabric for PPE applications. We have fabricated a nanofibrous silk-PEO material using electrospinning followed by zinc oxide thin film deposition by employing the atomic layer deposition technique. The composite fabric has shown 85% more antibacterial activity than the control fabric and was found to possess substantial superoxide dismutase-mimetic activity. The composite was further subjected to antiviral testing using two different respiratory tract viruses: coronavirus (OC43: enveloped) and rhinovirus (RV14: non-enveloped). We report a 95% reduction in infectious virus for both OC43 and RV14 from an initial load of ∼1 × 105 (sample size: 6 mm dia. disk), after 1 h of white light illumination. Furthermore, with 2 h of illumination, ∼99% reduction in viral infectivity was observed for RV14. High activity in a relatively small area of fabric (3.5 × 103 viral units per mm2) makes this antiviral fabric ideal for application in masks/PPE, with an enhanced ability to prevent antimicrobial infection overall.

SnO2 Nanostructured Thin Films for Room-Temperature Gas Sensing of Volatile Organic Compounds.

We demonstrated room-temperature gas sensing of volatile organic compounds (VOCs) using SnO2 nanostructured thin films grown via the aerosol chemical vapor deposition process at deposition temperatures ranging from 450 to 600 °C. We investigated the film's sensing response to the presence of three classes of VOCs: apolar, monopolar, and biopolar. The synthesis process was optimized, with the most robust response observed for films grown at 550 °C as compared to other temperatures. The role of film morphology, exposed surface planes, and oxygen defects were explored using experimental techniques and theoretical calculations to improve the understanding of the room-temperature gas sensing mechanism, which is proposed to be through the direct adsorption of VOCs on the sensor surface. Overall, the improved understanding of the material characteristics that enable room-temperature sensing gained in this work will be beneficial for the design and application of metal oxide gas sensors at room temperature.

Cationically Substituted Bi0.7Fe0.3OCl Nanosheets as Li Ion Battery Anodes.

Cation substitution of Bi3+ with Fe3+ in BiOCl leads to the formation of ionically layered Bi0.7Fe0.3OCl nanosheets. The synthesis follows a hydrolysis route using bismuth(III) nitrate and iron(III) chloride, followed by postannealing at 500 °C. Room temperature electrical conductivity improves from 6.11 × 10-8 S/m for BiOCl to 6.80 × 10-7 S/m for Bi0.7Fe0.3OCl. Correspondingly, the activation energy for electrical conduction reduces from 862 meV for pure BiOCl to 310 meV for Bi0.7Fe0.3OCl. These data suggest improved charge mobility in Bi0.7Fe0.3OCl nanosheets. Density functional theory calculations confirm this behavior by predicting a high density of states near the Fermi level for Bi0.7Fe0.3OCl. The improvement in electrical conductivity is exploited in the electrochemical performance of Bi0.7Fe0.3OCl nanosheets. The insertion capacity of Li+ ions shows an increase of 2.5×, from 215 mAh·.g-1 for undoped BiOCl to 542 mAh·g-1 for Bi0.7Fe0.3OCl after 50 cycles at a current density of 50 mA·g-1. Thus, the direct substitution of Bi3+ sites with Fe3+ in BiOCl results in nanosheets of an ionically layered ternary semiconductor compound which is attractive for Li ion battery anode applications.

Nanoscale Matrix Topography Influences Microscale Cell Motility through Adhesions, Actin Organization, and Cell Shape.

Mammalian cells are exposed to complex microenvironments of varying micro- and nanoscale structural features. These multiscale extracellular cues dictate important aspects of cell behavior, including migration, proliferation and differentiation. In this study, we fabricated anodized aluminum oxide (AAO) membranes of either 80 or 40 nm pore diameters. We utilized these membranes as extracellular matrix scaffolds to culture NIH-3T3 fibroblast cells and investigated how the surface nanotopography might regulate their motility. We observed faster and more persistent fibroblast migration on AAO membranes with larger pores. Through various cell-matrix interaction markers, we found that the surfaces with higher nanoporosity enhance motility through larger focal adhesions, aligned actin fibers, and polarized cell morphology. Our findings reveal the importance of nanoscale topographical cues present in the matrix environment in regulating submicrometer-scale subcellular mechanisms of stress fiber organization and adhesion formation, micrometer-scale cell-matrix interactions, and cell motility over hundreds of micrometers.

Indirect Phase Transformation of CuO to Cu2O on a Nanowire Surface.

The reduction of CuO nanowires (NWs) to Cu2O NWs undergoes an indirect phase transformation on the surface: from single crystalline CuO, to a disordered Cu2-δO phase, and then to crystalline Cu2O. A 9-12 nm disordered Cu2-δO is formed on the NW surface by exposing CuO NWs to CO at 1 Torr, 300 °C for 30 min. After 60 min, this layer decreases to 2-3 nm and is eliminated after 180 min. Energy dispersive X-ray spectroscopy using a scanning tunneling electron microscope and across a single NW reveals the disordered layer to be O-rich with respect to Cu2O with a maximum at. % Cu:O = 1.8. X-ray photoelectron spectroscopy shows adsorbed CO on the surface as evidence of the reduction reaction. Micro-Raman spectroscopy tracks the transformation in NWs as a function of reduction time. A CO enabled surface reduction reaction coupled to diffusion-limited transport of "nonlattice" O to the surface is proposed as a mechanism for Cu2-δO formation. The initial buildup of out-diffusing O to the surface appears to aid the formation of the disordered surface layer. The transformation follows Ostwald-Lussac's law which predicts formation of unstable phases over stable phases, when phase transformation rates are limited by kinetic or diffusional processes. The study provides a generalized approach for facile growth of few nanometer transient layers on multivalent, metal oxide NW surfaces.

Surface engineered CuO nanowires with ZnO islands for CO2 photoreduction.

Large arrays of massively parallel (10(8) cm(-2)) CuO nanowires were surface engineered with dense ZnO islands using a few pulsed cycles of atomic layer deposition (ALD). These nanowires were subjected to UV-vis radiation-based CO2 photoreduction under saturated humidity (CO2 + H2O mixture) conditions. We monitored CO2 to CO conversion, indicating the viability of these nanostructures as potential photocatalysts. High-resolution transmission electron microscopy and atomic force microscopy indicated an island growth mechanism of ZnO epitaxially depositing on pristine, single crystal CuO nanowire surface. Photoluminescence and transient absorption spectroscopy showed a very high density of defects on these ZnO islands which trapped electrons and enhanced their lifetimes. Peak CO conversion (1.98 mmol/g-cat/hr) and quantum efficiency (0.0035%) were observed in our setup when the ZnO islands impinged each other at 1.4 nm (8 cycles of ALD) diameter; at which point ZnO island perimeter lengths maximized as well. A mechanism whereby simultaneous H2O oxidation and CO2 reduction occurred in the active perimeter region between CuO nanowire and ZnO islands is proposed to explain the observed photoconversion of CO2 to CO.

Anodization control for barrier-oxide thinning and 3D interconnected pores and direct electrodeposition of nanowire networks on native aluminium substrates.

Here we report a strategy for combining techniques for pore branching and barrier layer thinning to produce 3D porous anodized aluminum oxide films with direct ohmic contact to the native aluminum. This method provides an example of a rationally designed template which need not be removed from the aluminum, but which is also not constrained to traditional 2D pore geometry. We first demonstrate the barrier layer removal and pore branching techniques independently, and then combine them to produce free standing arrays of interconnected Ni nanostructures. Nickel nanostructures are deposited directly onto the aluminum to demonstrate the success of the structural modification, and showcase the potential for these films to be used as templates. This approach is the first to demonstrate the design and execution of multiple pore modification techniques in the same membrane, and demonstrates the first directly deposited 3D structures on aluminum substrates.

Electrical conductivity of p-type BiOCl nanosheets.

High quality BiOCl nanosheets were fabricated using facile, room temperature hydrolysis of Bi(NO3)3 and HCl. The resulting nanosheets had dimensions of 500 nm with the exposed {001} facet. The band gap of the nanosheets was found to be 3.34 eV with conduction and valence band edges at -3.63 eV and -6.97 eV with respect to vacuum, respectively. The electrical conductivity of drop-cast BiOCl nanosheets was measured between aluminum patterned electrodes as a function of temperature and oxygen partial pressure (pO2). The activation energy for conduction in BiOCl was found to be 862 meV in the temperature range of 300-425 K and below 1000 mbar. The electrical conductivity varied with pO2, indicating σ ∝ pO2(1/4.05) and σ ∝ pO2(1/32) for low and sub atmospheric pressures, respectively. A prototypical device for low temperature (425 K) O2 sensing was demonstrated.

Plasmonic metal-to-semiconductor switching in Au nanorod-ZnO nanocomposite films.

We demonstrate conductivity switching from a metal to semiconductor using plasmonic excitation and charge injection in Au-nanorod (AuNRs)-ZnO nanocomposite films. ZnO films 12.6, 20.3, and 35.6 nm were deposited over AuNRs using atomic layer deposition. In dark conditions, the films transitioned from metallic to semiconducting behavior between 150 and 200 K. However, under sub-bandgap, white light illumination, all films behaved as semiconductors from 80 to 320 K. Photoresponse (light/dark conductivity) was strongly dependent on the thickness of ZnO, which was 94.4 for AuNR-12.6 nm ZnO and negligible for AuNR-35.6 nm ZnO. Conductivity switching and thickness dependence of photoresponse were attributed to plasmonically excited electrons injected from AuNRs into ZnO. Activation energies for conduction were extracted for these processes.

High to ultra-high power electrical energy storage.

High power electrical energy storage systems are becoming critical devices for advanced energy storage technology. This is true in part due to their high rate capabilities and moderate energy densities which allow them to capture power efficiently from evanescent, renewable energy sources. High power systems include both electrochemical capacitors and electrostatic capacitors. These devices have fast charging and discharging rates, supplying energy within seconds or less. Recent research has focused on increasing power and energy density of the devices using advanced materials and novel architectural design. An increase in understanding of structure-property relationships in nanomaterials and interfaces and the ability to control nanostructures precisely has led to an immense improvement in the performance characteristics of these devices. In this review, we discuss the recent advances for both electrochemical and electrostatic capacitors as high power electrical energy storage systems, and propose directions and challenges for the future. We asses the opportunities in nanostructure-based high power electrical energy storage devices and include electrochemical and electrostatic capacitors for their potential to open the door to a new regime of power energy.

MnO2/TiN heterogeneous nanostructure design for electrochemical energy storage.

MnO(2)/TiN nanotubes are fabricated using facile deposition techniques to maximize the surface area of the electroactive material for use in electrochemical capacitors. Atomic layer deposition is used to deposit conformal nanotubes within an anodic aluminium oxide template. After template removal, the inner and outer surfaces of the TiN nanotubes are exposed for electrochemical deposition of manganese oxide. Electron microscopy shows that the MnO(2) is deposited on both the inside and outside of TiN nanotubes, forming the MnO(2)/TiN nanotubes. Cyclic voltammetry and galvanostatic charge-discharge curves are used to characterize the electrochemical properties of the MnO(2)/TiN nanotubes. Due to the close proximity of MnO(2) with the highly conductive TiN as well as the overall high surface area, the nanotubes show very high specific capacitance (662 F g(-1) reported at 45 A g(-1)) as a supercapacitor electrode material. The highly conductive and mechanically stable TiN greatly enhances the flow of electrons to the MnO(2) material, while the high aspect ratio nanostructure of TiN creates a large surface area for short diffusion paths for cations thus improving high power. Combining the favourable structural, electrical and energy properties of MnO(2) and TiN into one system allows for a promising electrode material for supercapacitors.

Confined propagation of covalent chemical reactions on single-walled carbon nanotubes.

Covalent chemistry typically occurs randomly on the graphene lattice of a carbon nanotube because electrons are delocalized over thousands of atomic sites, and rapidly destroys the electrical and optical properties of the nanotube. Here we show that the Billups-Birch reductive alkylation, a variant of the nearly century-old Birch reduction, occurs on single-walled carbon nanotubes by defect activation and propagates exclusively from sp(3) defect sites, with an estimated probability more than 1,300 times higher than otherwise random bonding to the 'π-electron sea'. This mechanism quickly leads to confinement of the reaction fronts in the tubular direction. The confinement gives rise to a series of interesting phenomena, including clustered distributions of the functional groups and a constant propagation rate of 18 ± 6 nm per reaction cycle that allows straightforward control of the spatial pattern of functional groups on the nanometre length scale.

Profile evolution for conformal atomic layer deposition over nanotopography.

The self-limiting reactions which distinguish atomic layer deposition (ALD) provide ultrathin film deposition with superb conformality over the most challenging topography. This work addresses how the shapes (i.e., surface profiles) of nanostructures are modified by the conformality of ALD. As a nanostructure template, we employ a highly scalloped surface formed during the first anodization of the porous anodic alumina (PAA) process, followed by removal of the alumina to expose a scalloped Al surface. SEM and AFM reveal evolution of surface profiles that change with ALD layer thickness, influenced by the way ALD conformality decorates the underlying topography. The evolution of surface profiles is modeled using a simple geometric 3D extrusion model, which replicates the measured complex surface topography. Excellent agreement is obtained between experimental data and the results from this model, suggesting that for this ALD system conformality is very high even on highly structured, sharp features of the initial template surface. Through modeling and experimentation, the benefits of ALD to manipulate complex surface topographies are recognized and will play an important role in the design and nanofabrication of next generation devices with increasingly high aspect ratios as well as nanoscale features.

Plasmon-induced electrical conduction in molecular devices.

Metal nanoparticles (NPs) respond to electromagnetic waves by creating surface plasmons (SPs), which are localized, collective oscillations of conduction electrons on the NP surface. When interparticle distances are small, SPs generated in neighboring NPs can couple to one another, creating intense fields. The coupled particles can then act as optical antennae capturing and refocusing light between them. Furthermore, a molecule linking such NPs can be affected by these interactions as well. Here, we show that by using an appropriate, highly conjugated multiporphyrin chromophoric wire to couple gold NP arrays, plasmons can be used to control electrical properties. In particular, we demonstrate that the magnitude of the observed photoconductivity of covalently interconnected plasmon-coupled NPs can be tuned independently of the optical characteristics of the molecule-a result that has significant implications for future nanoscale optoelectronic devices.

Nanotubular metal-insulator-metal capacitor arrays for energy storage.

Nanostructured devices have the potential to serve as the basis for next-generation energy systems that make use of densely packed interfaces and thin films. One approach to making such devices is to build multilayer structures of large area inside the open volume of a nanostructured template. Here, we report the use of atomic layer deposition to fabricate arrays of metal-insulator-metal nanocapacitors in anodic aluminium oxide nanopores. These highly regular arrays have a capacitance per unit planar area of approximately 10 microF cm-2 for 1-microm-thick anodic aluminium oxide and approximately 100 microF cm-2 for 10-microm-thick anodic aluminium oxide, significantly exceeding previously reported values for metal-insulator-metal capacitors in porous templates. It should be possible to scale devices fabricated with this approach to make viable energy storage systems that provide both high energy density and high power density.

TEM-based metrology for HfO2 layers and nanotubes formed in anodic aluminum oxide nanopore structures.

Nanotubes are fabricated by atomic layer deposition (ALD) into nanopore arrays created by anodic aluminum oxide (AAO). A transmission electron microscopy (TEM) methodology is developed and applied to quantify the ALD conformality in the nanopores (thickness as a function of depth), and the results are compared to existing models for ALD conformality. ALD HfO2 nanotubes formed in AAO templates are released by dissolution of the Al2O3, transferred to a grid, and imaged by TEM. An algorithm is devised to automate the quantification of nanotube wall thickness as a function of position along the central axis of the nanotube, by using a cylindrical model for the nanotube. Diffusion-limited depletion occurs in the lower portion of the nanotubes and is characterized by a linear slope of decreasing thickness. Experimentally recorded slopes match well with two simple models of ALD within nanopores presented in the literature. The TEM analysis technique provides a method for the rapid analysis of such nanostructures in general, and is also a means to efficiently quantify ALD profiles in nanostructures for a variety of nanodevice applications.